The present invention relates to hydraulic fracturing operations, and more specifically to methods for identifying an induced subterranean formation fracture using neutron emission-based logging tools.
In order to more effectively produce hydrocarbons from downhole formations, and especially in formations with low porosity and/or low permeability, induced fracturing (called “frac operations”, “hydraulic fracturing”, or simply “fracing”) of the hydrocarbon-bearing formations has been a commonly used technique. In a typical frac operation, fluids are pumped downhole under high pressure, causing the formations to fracture around the borehole, creating high permeability conduits that promote the flow of the hydrocarbons into the borehole. These frac operations can be conducted in horizontal and deviated, as well as vertical, boreholes, and in either intervals of uncased wells, or in cased wells through perforations.
In cased boreholes in vertical wells, for example, the high pressure fluids exit the borehole via perforations through the casing and surrounding cement, and cause the formations to fracture, usually in thin, generally vertical sheet-like fractures in the deeper formations in which oil and gas are commonly found. These induced fractures generally extend laterally a considerable distance out from the wellbore into the surrounding formations, and extend vertically until the fracture reaches a formation that is not easily fractured above and/or below the desired frac interval. The directions of maximum and minimum horizontal stress within the formation determine the azimuthal orientation of the induced fractures. Normally, if the fluid, sometimes called slurry, pumped downhole does not contain solids that remain lodged in the fracture when the fluid pressure is relaxed, then the fracture re-closes, and most of the permeability conduit gain is lost.
These solids, called proppants, are generally composed of sand grains or ceramic particles, and the fluid used to pump these solids downhole is usually designed to be sufficiently viscous such that the proppant particles remain entrained in the fluid as it moves downhole and out into the induced fractures. Prior to producing the fractured formations, materials called “breakers”, which are also pumped downhole in the frac fluid slurry, reduce the viscosity of the frac fluid after a desired time delay, enabling these fluids to be easily removed from the fractures during production, leaving the proppant particles in place in the induced fractures to keep them from closing and thereby substantially precluding production fluid flow therethrough.
The proppants may also be placed in the induced fractures with a low viscosity fluid in fracturing operations referred to as “water fracs”. The fracturing fluid in water fracs is water with little or no polymer or other additives. Water fracs are advantageous because of the lower cost of the fluid used. Also when using cross-linked polymers, it is essential that the breakers be effective or the fluid cannot be recovered from the fracture effectively restricting flow of formation fluids. Water fracs, because the fluid is not cross-linked, do not rely on effectiveness of breakers.
Proppants commonly used are naturally occurring sands, resin coated sands, and ceramic proppants. Ceramic proppants are typically manufactured from naturally occurring materials such as kaolin and bauxitic clays, and offer a number of advantages compared to sands or resin coated sands principally resulting from the compressive strength of the manufactured ceramics and their highly spherical particle configuration.
Although induced fracturing has been a highly effective tool in the production of hydrocarbon reservoirs, there is nevertheless usually a need to determine the interval(s) that have been fractured after the completion of the frac operation. It is possible that there are sub-intervals within the desired fracture interval(s) which were ineffectively fractured, either due to anomalies within the formation or problems within the borehole, such as ineffective or blocked perforations. It is also desirable to know if the fractures extend vertically across the entire desired fracture interval(s), and also to know whether or not any fracture(s) may have extended vertically outside the desired interval. In the latter case, if the fracture has extended into a water-bearing interval, the resulting water production would be highly undesirable. In all of these situations, knowledge of the location of both the fractured and unfractured intervals would be very useful for planning remedial operations in the subject well and/or in utilizing the information gained for planning frac jobs on future candidate wells.
There have been several methods used in the past to help locate the successfully fractured intervals and the extent of the fractures in frac operations. For example, acoustic well logs have been used. Acoustic well logs are sensitive to the presence of fractures, since fractures affect the velocities and magnitudes of compressional and shear acoustic waves traveling in the formation. However, these logs are also affected by many other parameters, such as rock type, formation porosity, pore geometry, borehole fluid, borehole conditions, and presence of natural fractures in the formation. Another previously utilized acoustic-based fracture detection technology is the use of “crack noise”, wherein an acoustic transducer placed downhole immediately following the frac job actually “listens” for signals emanating from the fractures as they close after the frac pressure has been relaxed. This technique has had only limited success due to: (1) the logistical and mechanical problems associated with having to have the sensor(s) in place during the frac operation, since the sensor has to be activated almost immediately after the frac operation is terminated, and (2) the technique utilizes the sound generated as fractures close, therefore effective fractures, which are the ones that have been propped open to prevent closure thereof, often do not generate noise signals as easy to detect as the signals from unpropped fractures, which can generate misleading results.
Arrays of tilt meters at the surface have also been previously utilized to determine the presence of subterranean fractures. These sensors can detect very minute changes in the contours of the earth's surface above formations as they are being fractured, and these changes across the array can often be interpreted to locate fractured intervals. This technique is very expensive to implement, and does not generally have the vertical resolution to be able to identify which sub-intervals within the frac interval have been fractured and which have not, nor can this method effectively determine if the fracture has extended vertically outside the desired vertical fracture interval(s).
Microseismic tools have also been previously utilized to map fracture locations and geometries. In this fracture location method, a microseismic array is placed in an offset well near the well that is to be hydraulically fractured. During the frac operations the microseismic tool records microseisms that result from the fracturing operation. By mapping the locations of the mictoseisms it is possible to estimate the height and length of the induced fracture. However, this process is expensive and requires a nearby available offset well.
Other types of previously utilized fracture location detection techniques employ nuclear logging methods. A first such nuclear logging method uses radioactive materials which are mixed at the well site with the proppant and/or the frac fluid just prior to the proppant and/or frac fluid being pumped into the well. After such pumping, a logging tool is moved through the wellbore to detect and record gamma rays emitted from the radioactive material previously placed downhole, the recorded radioactivity-related data being appropriately interpreted to detect the fracture locations. A second previously utilized nuclear logging method is performed by pumping one or more stable isotopes downhole with the proppant in the frac slurry, such isotope material being capable of being activated (i.e., made radioactive) by a neutron-emitting portion of a logging tool run downhole after the fracing process. A spectroscopic gamma ray detector portion of the tool detects and records gamma rays from the resulting decay of the previously activated “tracer” material nuclei as the tool is moved past the activated material. The gamma spectra are subsequently analyzed to identify the activated nuclei, and thus the frac intervals. One or both of these previously utilized nuclear-based techniques for locating subterranean fractures has several known limitations and disadvantages which include:                1. The need to pump radioactive material downhole or to create radioactivity downhole by activating previously non-radioactive material within the well;        2. A requirement for complex and/or high resolution gamma ray spectroscopy detectors and spectral data analysis methods;        3. Undesirably shallow depth of fracture investigation capability;        4. Possible hazards resulting from flowback to the surface of radioactive proppants or fluids;        5. Potential for radioactivity contamination of equipment at the well site;        6. The need to prepare the proppant at the well site to avoid an undesirable amount of radioactive decay of proppant materials prior to performance of well logging procedures;        7. The possibility of having excess radioactive material on the surface which cannot be used at another well;        8. The requirement for specialized logging tools which are undesirably expensive to run;        9. The requirement for undesirably slow logging tool movement speeds through the wellbore; and        10. The need for sophisticated gamma ray spectral deconvolution or other complex data processing procedures.        
As can be seen from the foregoing, a need exists for subterranean fracture location detection methods which alleviate at least some of the above-mentioned problems, limitations and disadvantages associated with previously utilized fracture location detection techniques as generally described above.